Summing amplifier/ Inverting adder circuit using op amp 741

Summing amplifier or an adder is used to sum two signal voltages. Voltage adder circuit is a simple circuit that enables you to add several signals together. It has wide variety of applications in electronic circuits. For example, on a precision amplifier, you may need to add a small voltage to cancel the offset error of the op amp itself. An audio mixer is another good example of adding waveforms (sounds) together from different channels (vocals, instruments) before sending the combined signal to a recorder.
You can change the gain or add another input without messing up with the gains of other inputs. Just remember that the inverting summing amplifier circuit inverts the input signals. That’s not a big deal. If you need the opposite polarity, all you have to do is to put an inverting stage before or after the summer.

Circuit diagram

Components required

Power supply

IC 741 Op Amp

Resistors (100kx4)

Output waveform

Adding one sine wave and DC voltage

Adding two different waveforms

Working

Here the input voltages V1, V2, V3 are given in to adder circuit.

This is an inverting summing amplifier because output is the sum of inputs with a sign change.

To construct a non inverting adder, you can cascade one ‘Inverting amplifier’ with unity gain along with this circuit.

Output of this adder circuit is given by – (V1+V2+V3).

Consider the current flowing through the input resistors are

Then by Kirchof’s current law, the current flowing through feedback resistor Rf is given by the sum of these 3 currents.

This current will flows through the feedback resistor Rf, because the point ‘K’ acts as virtual ground point. So the voltage drop at Rf is given by

-ve sign is due to the op amp connected in inverting mode.

This circuit is called a summing ‘amplifier’ because it can provide gain. By adjusting the value of Rf the gain can be changed.